A light-emitting device is a device to emit light to outside by converting electric energy into light energy. As an example of such light-emitting device, there is a light-emitting diode (LED).
Recently, gallium nitride (GnN)-based LEDs are implemented as LEDs of red, green and blue (RGB) which can implement high brightness and white light owing to a metal organic chemical vapor deposition (MOCVD) of GaN, a growth method such as a molecular-beam epitaxial growth (MBE), and novel processing technology.
Such GnN-based LEDs are applied to a display, a backlight unit, an indoor/outdoor lighting device, etc. As the GnN-based LEDs are applied to an optical source for illumination, concern over LEDs of high output is increased.
The conventional LEDs of a horizontal structure have a low thermal conductivity because they are formed on an insulating substrate such as a sapphire substrate. Further, in the conventional LEDs of a horizontal structure, a process of removing part of an active layer is required for an ohmic contact between the active layer and an electrode, since an electrode is formed in a horizontal direction. This may cause a light-emitting area to be reduced, and a forward voltage may be increased due to current crowding.
In order to solve such problems, LEDs of a vertical structure is being spotlighted.
In the LEDs of a vertical structure, two electrodes and electrode pads are positioned at upper and lower sides of the LEDs as a sapphire substrate is removed by a laser lift-off (LLF) process. Under such structure, a current flows in one direction to enhance light-emitting efficiency and brightness.
However, in a case where the current is concentrated to a central part below an ‘n’-type electrode formed on an ‘n’-type semiconductor layer in the LEDs of a vertical structure, photons generated by recombination between electrons and holes in the active layer are concentrated to the central part below the ‘n’-type electrode. Then a large amount of the generated photons are absorbed to the ‘n’-type electrode. In this case, an effective light emission region, a region rather than the ‘n’-type electrode may be reduced, thereby lowering light-emitting efficiency and brightness of the LEDs of a vertical structure.
In order to solve such problems, has been developed a structure to form a current-blocking layer (an insulating region) by depositing an insulating material or injecting ions on/into part of a ‘p’-type semiconductor layer in the LEDs of a vertical structure.
Such current-blocking layer induces a current flow concentrated to a region below the ‘n’-type electrode in a vertical direction, to a horizontal direction (effective light emission region). Accordingly, light-emitting intensity in the effective light emission region is increased, and thus light-emitting efficiency and brightness are enhanced. However, the current-blocking layer reduces an ohmic contact area in the ‘p’-type semiconductor layer due to its insulating characteristic, resulting in increase of a forward voltage.